July 16th - Welcome to the late show.....

 Overview:

A fairly stagnant pattern for the next 48+ hours characterized by a diffuse upper-level high pressure system centered to our east/northeast in north central New Mexico. Flow around the western periphery of this high places southeastern Arizona in variations of easterly or southeasterly flow. Moisture values are relatively close to climatology with typical diurnal variations in dewpoints. CAMs are in general agreement that any activity in valley locations over the next 48 h will likely be near or after sunset events. This shift in timing to later events may be associated with a pattern characterized by larger values of convective inhibition stretching across Pima, Santa Cruz, and Cochise counties. Whether or not convection can work its way into the valley will depend on subtle features in the mid-level low and are difficult to discern ahead of time. As for today, the likelihood of convection in the greater Tucson area may hinge on an inverted trough near the international border as well as the development of convectively driven circulation in Cochise county. These features, in combination with a northward propagating outflows boundary from Santa Cruz, could aid in allowing storms to maintain/redevelop as they work across the higher terrain near sunset.

Today:

The large-scale pattern has not changed much over the past 24 h. The water vapor imagery and over-layed 500 hPa pattern show an anticyclone situated over New Mexico, with residual dry

10 AM MST Water Vapor and 500 hPa Mesoanalysis

air from an anticyclonic wave break stretching from eastern Texas into Chihauhau and extreme southeastern New Mexico. Potential help for convection today is sitting in Mexico in the form of a cyclonic circulation with an inverted trough stretching towards the northwest highlighted by the black line that I've drawn. The northern extension of this feature is the inverted trough alluded to in the overview.

Precipitable water distribution is fairly typical for this time of the year, with highest values stretching from the Gulf of California into southwestern portions of AZ, with values dissipating to the northeast.

                                     10 AM MST PWTR Mesoanalysis

Values across the metropolitan areas of Tucson as well as Phoenix are generally about 1.3-1.5" or close to climotological values.

Besides the cyclonic feature in Texas, the one other thing today that represents a change from recent conditions is the strength and coverage of the convective inhibition this morning.

                           10 AM MST MLCAPE and CINH Mesoanalysis

As of 10 AM, values of CIN exceeding 100 J/kg covered most of southern AZ with the highest values of 300 J/kg up towards Phoenix. This should suppress convective initiation until later in the day and may partially explain the CAMs insistence on later timing of convection into the greater Tucson metro area. 

The sounding from Tucson this morning sounding indicated over 300 J/kg of CIN, with minimal CAPE. Lapse rates in the 700-500 hPa layer are still fairly steep, but a weak inversion, with more moist adiabatic lapse rates above 500 hPa may limit CAPE values a bit.

                                          12Z KTUS Sounding

PWTR values have increased to 1.5" and the sounding would suggest a mixed layer surface dewpoint in the mid 50's F. Unfortunately, data issues are made it difficult to locate a sounding with a wind staff this morning, but if this figure had one, it would show mid-level flow of about 25 knots from the east/southeast which is a little more robust than yesterday.

The Forecast:

While all CAMs seem to agree that the threat of convection will be late, different initializations of the UAWRF and the HRRR have different ideas of the likelihood of precipitation this evening. Comparing the reflectivity patterns from the RR and HRRR intialized UAWRF from 12Z this morning, we can see that HRRR intialization has greater coverage of convection in eastern Pima and western Cochise counties, although there is better agreement in the activity further south over Santa Cruz. 

                   12Z UAWRF HRRR reflectivity forecast valid at 8 PM MST 

                   12Z UAWRF RR reflectivity forecast valid at 8 PM MST 

The difference in convection appears to be related to subtle but important differences in the 500 hPa flow, although how much of this is convective feed back is difficult to discern. The enhanced inverted trough in the 12Z HRRR intialization is likely directly  related to the probability of convection working its way into the valley.

                   12Z UAWRF HRRR 500 hPa forecast valid at 6:30 PM MST 

                          12Z UAWRF RR 500 hPa forecast valid at 6:30 PM MST 

Thermodynamic profiles heading into the evening are similar to the last few nights with MLCAPES of of 800-1000 J/kg and DCAPES of 1900 J/kg. Consequently, any storms that do develop will continue to carry a wind risk, with the SPC continuing to carry a marginal risk for severe winds in the area.

The CAM ensembles highlight both the post sunset convection threat and the limited probability of that threat. The ensembles show the potential for reflectivities exceeding 50 dBZ throughout most of eastern Pima county maximized in in the time period between 00-04Z (5-9 PM MST). However, the probability of a reflectivity greater than 40 dBZ  is approximately 40% in Tuscon with greater probabilities to the SE and lesser to the NW.

                   12Z HREF MAx 4hr reflectivity and prob > 40 dBZ valid at 9 PM MST 

The threat of excessive rainfall shifts from east to west this evening shifting from Santa Cruz and Western sections of Cochise counties in the 5-8 PM MST time frame, 

            12Z HREF Max 3 hr Precip and prob > 1" valid at 8 PM MST

to south central Pima county in the 8-11 PM timeframe

                12Z HREF Max 3 hr Precip and prob > 1" valid at 11 PM MST

The combination suggests that the probability of heavy rainfall is 20% or less in the Tucson metro this evening. 

Late Update: Latest runs of the HRRR indicate a lower probability of convection in association with lower CAPE values this evening as a result of surface dewpoints forecasts dropping to near 50 F. 

Tomorrow:

The pattern will essentially be on a repeat cycle for tomorrow. The 500 hPa flow will continue to be out of the southeast at about 20 knots....

                         12Z HREF 500 hPa heights/wind valid at 12 PM MST

although PWTR values may drop back slightly to 1.2-1.3"

                                        12Z HREF PWTR valid at 12 PM MST

CAPE values are expected to peak near 1000 J/kg in the afternoon,

                         12Z HREF MUCAPE valid at 2 PM MST

The ensembles indicate a more focused threat area near the NE corner of Pima county (the Catalina and White mountains) and south central Pima county. There is generally a 30% chance of greater than 40 dBZ throughout the area in the 5-9 PM.

               12Z HREF MAx 4hr reflectivity and prob > 40 dBZ valid at 9 PM MST

The threat of heavy rainfall amounts will be generally less than 10%, although at least one member is showing significant rainfall over the Catalinas.

          12Z HREF Max 3 hr Precip and prob > 1" valid at 8 PM MST